In the operation of gas turbines, particularly in electric power plants, various kinds of control systems have been employed from relay-pneumatic type systems, to analog type electronic controls, to digital controls, and more recently to computer based software controls. U.S. Pat. No. 4,308,463--Giras et al., assigned to the assignee of the present invention and incorporated herein by reference, lists several of such prior systems. That patent in particular discloses a digital computer based control system for use with gas turbine electric power plants. It will be noted that the Giras et al. patent is one of a family of patents all of which are cross referenced therein.
Subsequent to the Giras et al. patent, other control systems have been introduced by Westinghouse Electric Corporation of Pittsburgh, Pa. under the designations POWERLOGIC and POWERLOGIC II. Similar to the Giras et al. patent these control systems are used to control gas turbine electric power plants. However, such control systems are primarily micro-processor based computer systems, i.e. the turbine control systems are implemented in software, whereas prior control systems were implemented in electrical and electronic hardware. All modes of turbine-generator operation are controlled including control of fuel flow after the attainment of full load. Loss of full load presents interesting control system problems in relation to fuel flow control.
When load rejection occurs in a combustion turbine-generator arrangement, the initial problem is to control fuel flow in a manner which limits turbine overspeed to a value less than the turbine trip point. If turbine speed exceeds the turbine trip point, fuel flow ceases and a flame out will occur. If load rejection is caused by the opening of the generator circuit breaker, a condition which can be readily detected in relation to the state change of breaker auxiliary contacts, appropriate procedures have been provided in previous combustion turbine control systems to hold the turbine speed set point to synchronous speed for the control of fuel flow. Since the turbine is accelerating due to a loss of load, actual turbine speed will be above the turbine speed set point resulting in a closing of the fuel throttle valve by the control system.
A fuel problem can occur in previous procedures for controlling fuel flow in a generator breaker based load rejection condition. The problem stems from the use of a proportional, integral, differential (PID) controller. Since a PID controller is primarily a high gain proportional control, the greater the difference between actual turbine speed and the turbine speed set point, the further the closure of the fuel throttle that will occur. If the speed difference is great enough, flame out can occur. Prior procedures have been developed to prevent such a flame out condition from occurring in a generator breaker based load rejection situation. Unfortunately, load rejection in a gas turbine/generator arrangement has another form, namely, downline load rejection. Since downline load rejection is not as readily detectable as generator breaker based load rejection, a flame out situation due to turbine speed exceeding the turbine trip point is significantly more probable.
When downline load rejection occurs, two significant events take place. First, the generator breaker remains closed, which in previous control systems allows the speed reference to remain set to the speed equivalent of full turbine load versus synchronous speed. If it is assumed that synchronous speed, i.e., the speed desired for connection of the generator to an electrical power grid, is 3600 RPM and that the turbine generator has a 4% droop governor, i.e., 100% generator power is equivalent to 104% turbine speed, the speed equivalent of full load is 3744 RPM. Second, actual turbine speed drops to approximately 3600 RPM at the instant of load rejection while the speed reference remains at 3744 RPM. The result of these events is that the fuel throttle controller sensing the speed difference between actual turbine speed and the speed reference will cause the fuel throttle valve to open until actual turbine speed exceeds the speed reference, i.e., 3744 RPM. In such a situation, it is more probable that actual speed will exceed the turbine speed trip point.
A need still exists for a turbine-generator control scheme which will sense downline load rejection and maintain the turbine speed reference at a value which minimizes the occurrence of flame out conditions.
Although, the operation of a gas turbine electric power plant is described herein, it should be noted that the invention has broader application to the control of fuel in a gas turbine in a load rejection situation.